Tunable dielectric materials have a broad variety of electrical applications, such as in tunable phase shifters, bandpass and band reject filters and Q resonators. Tunable dielectric materials have a dielectric constant that is a function of the electric field applied to the material. By selectively biasing the material and then passing an electromagnetic signal through the material, a characteristic such as the signal's frequency and/or phase can be selectively controlled. Such devices are disclosed in U.S. Pat. Nos. 5,472,935 and 5,589,845, both of which are incorporated herein by reference in their entireties.
Dielectric materials can be formed as thin film and bulk dielectric materials, which have significantly differing electrical properties, even though they can have similar chemical compositions. The differing electrical properties result from substantial differences in the microstructures of the materials. The "microstructure" of a material refers to a combination of the material's grain size, grain shape, and internal stress.
The microstructure of a dielectric material is intrinsic to the technique used to fabricate the material. Thin film dielectric materials are typically formed on a substrate by creation processes, such as RF magnetron DC sputtering, sol-gel, liquid phase epitaxy, electron beam evaporation, thermal evaporation, laser ablation, metal-organic chemical vapor deposition, and chemical vapor deposition. In contrast, bulk dielectric materials are typically monolithic materials (i.e., free standing materials not formed on a substrate) by different creation processes, such as hot pressing, isostatic pressing, jiggering, extrusion, slip casting, band casting, calendaring, injection molding, Czochralski growth, vernevil growth, flame fusion and hydrothermal growth techniques. As a result of the differing deposition processes, thin films typically have orders of magnitude lesser thicknesses than bulk dielectric materials. Although thin film deposition processes can fabricate films up to 10 microns thick, typical film thicknesses range from about 0.05 to 3.0 microns.
Of the two distinct types of dielectric material, thin film material is preferred in constructing electrically tunable devices. Compared to bulk dielectric materials, thin film dielectric materials allow for relatively large electric fields using relatively low bias voltages.
In designing an acceptable electrically tunable thin film device, it is important to provide a high degree of tunability and a minimum of signal loss. Thus, the insertion loss for the device should be maintained as low as possible.